Tests for LinkedList methods

[jay-tyler]

Implement testing for all new methods. Let's follow TDD and finish this goal before implementing the methods.

* __add__()
* __iadd__()
* __append__()
* clear()
* __contains__()
* copy()
* count()
* __delitem__()
* extend()
* __getitem__()
* index()
* insert()
* __iter__()
* __len__()
* __mul__()
* __imul__()
* __rmul__()
* pop()
* remove()
* reverse()
* __reversed__()
* __setitem__()
* sort()

All the above should follow match the Python api for lists as closely as possible.